The present invention relates to a new process for producing styrene by the catalytic dehydrogenation of ethylbenzene into styrene in the presence of steam.
Styrene is widely used as raw material for the production of a large number of resins, plastics and elastomers, the extent of its use being mainly attributed to the suitability of styrene to be easily polymerized, for example, into polystyrene, or to be copolymerized, for example with butadiene, to produce rubbers.
The production of styrene, as regards the dehydrogenation process as well as the variety of catalysts used in this process, is well known. As present, the main research objective lies in the improvement of the profitability of the process. Especially, a process for the dehydrogenation of ethylbenzene according to which the dehydrogenation is carried out in a plant which comprises at least three dehydrogenation reactors in series, and intermediate heating devices for the reactors in which the reaction effluents are subjected to a heating by heat exchange with stream is known. According to this process, steam is first of all used for heating the reaction effluents and it is then mixed with ethylbenzene at the inlet of the first reactor. The operating conditions adopted in this process are the mixing of 3 to 10 moles of steam with 1 mole of ethylbenzene, and a temperature and a pressure at the inlet of the last reactor of 600.degree.-680.degree. C. and 0.4-0.8 kg/cm.sup.2, respectively. The temperature and pressure conditions at the inlet of the other reactors are 600.degree.-680.degree. C. and 2- 0.4 kg/cm.sup.2 (absolute), respectively, and the overall space velocity of ethylbenzene is equal to or greater than 0.44 h.sup.-1. By hourly space velocity of ethylbenzene is meant the ratio of the volume flowrate of ethylbenzene to the total volume of the catalyst. According to this process, an ethylbenzene conversion rate of the order of 65-75%, or even greater, and a molar selectivity of the order of 90% are obtained. However, the implementation of such a process, over a period of time, leads to a deterioration in the styrene selectivity as well as in the ethylbenzene conversion rate which may, nevertheless, be limited by increasing the reaction temperatures. In fact, gradual degradation of the catalyst and appearance of carbon deposits on this catalyst are observed over a period of time. Additionally, the presence of hot points along the production line (especially in the intermediate heating devices) gives rise to thermal degradation reactions producing heavy and choking compounds. It is estimated that the rate of heavy products formed is of the order of 17000 ppm, expressed per ton of effluent hydrocarbons, or even greater. In the end, an increase in charge loss, a decrease in the efficiency of the catalyst and, therefore, the need for frequent cleaning, results therefrom. With a view to eliminating the drawbacks of such a process, the Applicant Company has developed a new process for the dehydrogenation of ethylbenzene which makes it possible to obtain excellent ethylbenzene conversion rates, greater than 73%, and a molar selectivity greater than 93%.